Toxicology Brief: The critical care of aflatoxin-induced liver failure in dogs
This article focuses on the therapeutic management of canine aflatoxicosis, drawing from treatments used in other cases of hepatotoxicosis or hepatic failure.
In 1952, an outbreak of fatal liver disease in dogs occurred in the southeastern United States. The disease, termed hepatitis X, was characterized by icterus, lethargy, anorexia, petechiae, melena, epistaxis, and hematemesis. Affected dogs died one to 14 days after clinical presentation.1-4 The postmortem findings of hepatitis X were noted to be similar to those in swine and cattle after ingestion of moldy corn.5 Further investigation revealed that the disease resulted from exposure to aflatoxins produced by the fungus Aspergillus species. In 1959, the role of aflatoxins in hepatitis X was confirmed when a group of dogs acquired the disease after being fed purified aflatoxin B1.6
Since the identification of aflatoxins, poisonings have been described in many species, including people, dogs, poultry, swine, and other livestock. Aflatoxicosis inflicts varying degrees of hepatic damage depending on the species, exposure (dose and duration), and the animal's nutritional status (protein and vitamin intake).7-10 Of the four natural aflatoxins—B1, B2, G1, and G2—aflatoxin B1 is the most toxic. In addition to having mutagenic and immunosuppressive effects, it is a potent carcinogen that has been linked to hepatic cell carcinoma in people.9,11
Studies on aflatoxicosis in dogs have focused on its diagnosis and the pathologic changes that occur after aflatoxin exposure. No antidotes for the toxins exist, and few studies have addressed the care and management of dogs with aflatoxicosis. This article focuses on the therapeutic management of canine aflatoxicosis, drawing from treatments used in other cases of hepatotoxicosis or hepatic failure.
ROUTE OF EXPOSURE
Most cases of aflatoxicosis occur when animals ingest moldy corn, peanuts, or other foods susceptible to contamination by Aspergillus flavus or Aspergillus parasiticus.7,8,12 Preventive measures can be taken by pet food manufacturers through careful monitoring of aflatoxin concentrations in food and proper storage of food in a clean, dry area.
Outbreaks of aflatoxicosis continue to occur. In December 2005, several acute outbreaks of hepatic failure in dogs across the United States raised suspicion of a food-related toxicosis. Postmortem examination of the affected dogs revealed severe liver damage suggestive of aflatoxicosis. Affected dogs consumed food produced by a single manufacturer. Analysis of the pet food confirmed contamination with aflatoxins. The manufacturer issued a recall, and the FDA released an alert, but over the course of the next few weeks more than 100 dogs became intoxicated and died.13,14
Dogs with aflatoxicosis often have a vague initial presentation because of the insidious onset of the disease. Signs include anorexia, lethargy, vomiting, and jaundice. Hematochezia, melena, and hematemesis are sometimes present, as well as mucosal or more widespread petechiae and ecchymoses. Patients may have peripheral edema or ascites. Polyuria and polydipsia may also be noted. In some cases of toxicity, acute death occurs before clinical signs are noted. The disease is progressive, and the case fatality rate is high.
The signs of aflatoxicosis are consistent with liver failure; they are not specific for an aflatoxin etiology.4,13,15-18 Other differential diagnoses for acute liver failure include drug toxicosis (e.g. acetaminophen, ketoconazole, carprofen), biotoxins (e.g. blue-green algae, Amanita species mushrooms, cycad palm seed), heavy metal toxicosis, infectious diseases (e.g.Clostridium species infection, infectious canine hepatitis, systemic fungal infections), and massive ischemia (e.g. arterial or venous occlusion).
CBC, SERUM CHEMISTRY, AND CLOTTING PROFILE FINDINGS
Patients may have mild to moderate anemia and, in cases of disseminated intravascular coagulation (DIC), may be thrombocytopenic. Hematologic parameters are otherwise unremarkable. Liver enzyme activities (alanine transaminase, alkaline phosphatase, aspartate transaminase, gamma-glutamyltransferase ) and total bilirubin concentrations are elevated, and cholesterol and albumin concentrations are decreased. Coagulation profiles are often markedly abnormal, with prolonged activated partial thromboplastin times and prothrombin times and decreased antithrombin and protein C concentrations.4,13,18
HEPATIC HISTOLOGIC LESIONS
Histologic examination of liver biopsy samples reveals hepatocellular fatty vacuolation, hepatic necrosis, periportal necrosis, portal fibrosis, and perivenular necrosis and inflammation. Bile duct proliferation is present, and bile canaliculi may be plugged with bile casts.4,13,15-18
FOOD OR TISSUE ANALYSIS
Consider aflatoxicosis in any dog presenting with signs of acute liver failure, especially when the history is suggestive of a toxin (e.g. multiple dogs in same household affected, recent change in food). Since clinical signs are nonspecific for aflatoxicosis, a definitive diagnosis requires analyzing suspect food or identifying toxins in tissue samples. Several veterinary diagnostic laboratories offer qualitative or quantitative tests for aflatoxin in feed. Tissue analysis is less available. A list of accredited diagnostic laboratories is provided online by the American Association of Veterinary Laboratory Diagnosticians at www.aavld.org/mc/page.do?sitePageId=33930. Aflatoxins can be identified in urine, but metabolites are quickly cleared from the body, making postexposure detection difficult.19
Because no specific antidote for aflatoxins is available, treatment is aimed at supportive care and the management of specific hematologic and biochemical derangements (Table 1).
Table 1: Treatment Considerations in Dogs with Aflatoxicosis
Aflatoxins typically do not produce signs of toxicosis until after they have been absorbed from the gastrointestinal tract. So unless the dog has a recent history of ingestion of a known aflatoxin-contaminated food (which is rarely the case), emetics and other gastrointestinal decontamination methods are not indicated. Once absorbed, aflatoxins are rapidly metabolized in the liver. Interaction with cytochrome p450 results in toxic metabolites that are then conjugated with glutathione (GSH). Dose- dependent hepatocellular injury occurs from the production of free radicals that cause oxidative damage. Epoxide metabolites of aflatoxins can also bind and disrupt DNA, leading to hepatocellular carcinoma. The potential for damage from aflatoxins is long-term; toxins may remain bound to circulating albumin for up to two months.10,20,21
While specific antidotes for aflatoxins have not been found, several treatments may augment the metabolism and excretion of the toxins. Detoxification of the damaging aflatoxin epoxide occurs via conjugation with hepatic GSH10,21,22 ; therefore, administering medications that promote hepatic GSH synthesis may increase a dog's capacity to metabolize and excrete aflatoxins. Medications and vitamins that affect GSH stores or that have hepatoprotective effects include
- N-acetylcysteine. N-acetylcysteine is thought to replenish GSH stores by acting as a GSH substitute and precursor. In addition, N-acetylcysteine has beneficial effects on liver blood flow, oxygen extraction, and the formation of nonglutathione products that protect against cell injury.23 Experimentally, N-acetylcysteine has protective effects against aflatoxin damage in chickens, rats, and rabbits.24-26
A 5% solution (the 20% solution can be diluted 1:4 with sterile water or saline solution) is administered intravenously (over 20 minutes) or orally. Products not labeled for intravenous use require a 0.2-μm syringe filter. A loading dose of 140 mg/kg is given intravenously once followed by a maintenance dose of 50 to 70 mg/kg given intravenously every four to six hours for one to three days.27
- S-adenosylmethionine (SAMe). SAMe is a supernutrient (a nutrient that must be provided exogenously when endogenous synthesis is impaired or insufficient) used to increase the availability of precursors for GSH synthesis. It also acts as a methyl donor and enzyme activator for key reactions that maintain membrane structure and function. SAMe has been shown to decrease cholestasis and fibrosis in people and nonhuman primates with liver disease.28
The recommended dosage in dogs is 17 to 20 mg/kg given orally once a day as needed for weeks to months.29,30 Denosyl (Nutramax Laboratories) and Zentonil (Vétoquinol) are SAMe veterinary products that each offer three tablet sizes. Ideally, these products should be given on an empty stomach to increase absorption.31,32
- Milk thistle (silymarin). The silybin-phosphatidylcholine complex in silymarin enhances GSH production and prevents bioactivation of certain toxins. Silymarin may also protect normal hepatocytes, promote the regeneration of hepatocytes, and act as a free-radical scavenger and antioxidant.33 Specifically, silymarin has been reported to have hepatoprotective effects against aflatoxicosis in chickens and rats.34,35
A standard therapeutic dosage for silymarin has not been established in dogs; recommendations range from 7 to 15 mg/kg/day (based on human data) to 50 to 150 mg/kg/day (based on Amanita phalloides hepatotoxicoisis in dogs).30 Nutramax Laboratories offers the oral veterinary product Marin (silybin/vitamin E/zinc) in two tablet sizes that contain either 24 or 70 mg silybin. The product information for Marin indicates a low mg/kg dose of silybin; instructions are given for weight ranges with doses varying from about 1.5 to 4.5 mg/kg once a day depending on a dog's size.36 If a higher dose is desired, Marin would have to be administered extra-label. Although Marin also contains zinc and vitamin E, the zinc content in the product is low and the safety margin for vitamin E is high.
Denamarin (Nutramax Lab o ra tories) is also available and contains a combination of SAMe and silybin.
- Vitamin E. Vitamin E is a fat-soluble vitamin that may have a protective effect in aflatoxin exposure by counteracting the formation of lipid peroxides. Vitamin E supplementation in rabbits fed aflatoxin resulted in fewer and less severe changes in liver enzymes and other biochemical parameters as compared with a control group.37 Because of biliary obstruction, oral absorption of vitamin E may be decreased in aflatoxicosis.
The recommended dosage is 100 to 400 IU given orally twice a day.38 Marin contains vitamin E and comes in two tablet sizes that contain 105 or 300 IU vitamin E.36
- L-carnitine. L-carnitine is an amino acid derivative synthesized primarily by the liver. It is involved in fatty acid transportation and mitochondrial function. A study of chronic aflatoxicosis in quail found enhanced antioxidant effects with L-carnitine supplementation.39 L-carnitine helps mobilize fatty acids in cats with hepatic lipidosis, but it is unknown whether it has similar activity in dogs with aflatoxicosis.40 The recommended dosage in cats is 50 to 100 mg/kg given orally once a day for two to four weeks.41
- Others. Other nutraceuticals that may be beneficial in aflatoxin-induced liver disease include zinc, ursodeoxycholic acid (contraindicated in patients with biliary obstruction), phosphatidylcholine, and vitamins C and A.37,42
Patients with liver failure often require fluid therapy to treat dehydration from fluid loss, correct electrolyte and acid-base imbalances, lower serum ammonia concentrations, and reduce the risk of DIC. Hypovolemia and hypoperfusion are common in liver failure; contributing factors include hemorrhage from coagulation abnormalities, reduced colloidal osmotic pressure, peritoneal effusions, and water loss from vomiting or diarrhea. Initial resuscitation with colloids may be advisable (e.g. dextran-70, hetastarch, or fresh frozen plasma). Continuing fluid therapy in patients with acute liver failure may consist of isotonic crystalloids supplemented with electrolytes and dextrose as needed. Monitor the patient's acid-base status and electrolyte and blood glucose concentrations regularly, and adjust fluids accordingly. If hypernatremia is present, one-third to one-half strength saline solution is recommended.43
Control of hemorrhage
An immediate and life-threatening concern in dogs with aflatoxicosis is an increased bleeding tendency resulting from hepatocellular damage and decreased synthesis of coagulation factors, which creates a hypocoagulable state. Constant-rate infusions of fresh frozen plasma may be indicated to supplement coagulation factors. If the patient is moderately to severely anemic, fresh whole blood is preferred.4,13,43
In addition to decreased production of coagulation factors, a vitamin K deficiency may contribute to the coagulation deficiencies seen with aflatoxicosis. On histologic examination of liver samples from dogs with aflatoxicosis, bile canaliculi are sometimes plugged with casts. Biliary obstruction interferes with the bile-salt facilitated absorption of lipid-soluble vitamin K. Moreover, aflatoxin is a coumarin-like derivative and may have a direct inhibitory effect on vitamin K activity.4
Vitamin K is an essential cofactor in the activation of coagulation factors II, VII, IX, and X, as well as anticoagulant proteins C and S. Thus, a vitamin K deficiency may contribute to the hypocoagulable state in patients with aflatoxicosis. Treatment with vitamin K1 has been shown to improve, though not resolve, the coagulation abnormalities. Vitamin K should be given at a dose of 1 to 5 mg/kg either subcutaneously or orally, although subcutaneous administration is preferred because of decreased gastrointestinal absorption of oral vitamin K.44,45
Another reason for increased bleeding tendencies in patients with aflatoxicosis is enteric hemorrhage incited by portal hypertension. Histologic examination of tissue samples from dogs with aflatoxicosis reveals hepatic outflow occlusion and portal fibrosis, and the gastrointestinal tract shows resultant mucosal congestion, edema, and hemorrhage. Dogs with aflatoxicosis may exhibit hematochezia, melena, and hematemesis.4,13,15,17 Blood loss from the gastrointestinal tract and hemorrhagic effusions into the abdomen can lead to anemia.16,18 Packed red blood cells, bovine hemoglobin glutamer-200 (Oxyglobin—Biopure), or whole blood may be administered to increase oxygenation and blood volume, but these products will not decrease the portal hypertension.43,46
Finally, DIC has been reported in several cases of canine aflatoxicosis.18 The inciting cause of DIC in cases of acute hepatitis or liver failure is thought to be tissue procoagulants released into circulation from damaged hepatocytes.45 The combination of increased consumption of coagulation factors from DIC and reduced synthesis and activation of coagulation factors in aflatoxicosis may lead to severe hemorrhage. Fresh whole blood or fresh frozen plasma would be indicated in these cases, but unfortunately, patients with hemorrhagic signs have extremely poor prognoses.18
Because of a compromised gastrointestinal barrier in acute liver disease, patients with aflatoxicosis have an increased risk of bacteremia and sepsis. Using antibiotics directed toward gram-negative bacteria is suggested. Metronidazole can be used in combination with amoxicillin, ampicillin, cephalexin, or cefadroxil. Metronidazole is effective against gram-negative anaerobes that produce ammonia, so it may help prevent hepatoencephalopathy as well as sepsis.43 Because metronidazole is metabolized by the liver and has been reported to cause hepatotoxicosis, reducing the dosage by 50% (to 5 mg/kg twice a day) may be appropriate in patients with aflatoxicosis.47
Antiemetics and gastrointestinal protectants
Patients with aflatoxicosis frequently pre sent with a history of progressive vomiting. A constant-rate infusion of metoclopramide is a good choice for initially treating these animals. If the vomiting is refractory to treatment with metoclopramide, adding a serotonin antagonist such as dolasetron or ondansetron may provide relief. Also consider sucralfate, famotidine, and other gastrointestinal protectants in patients with aflatoxicosis to manage ulceration and decrease acid in the gastrointestinal tract.13,43
Control of hepatoencephalopathy
In patients showing signs of hepatoencephalopathy, medications such as lactulose, neomycin sulfate, metronidazole, and flumazenil may be warranted. More information on treating hepatoencephalopathy in patients with acute liver failure is found elsewhere.43
A high-protein diet combined with aflatoxin exposure has been associated with increased preneoplastic hepatic lesions in rats.48 However, while low-protein diets decrease hepatocarcinogenicity, they appear to predispose patients to a subacute toxicosis.49 Thus, recovering dogs should be fed a balanced canine diet. Avoid protein -restricted diets unless hepatoencephalopathy is suspected. In cases of hepatoencephalopathy, parenteral nutrition may be preferred.43
Frequently monitor the clinical signs; hepatic enzyme activities; electrolyte, cholesterol, and bilirubin concentrations; coagulation parameters; and hepatic function of patients with aflatoxicosis. Pay particular attention to any signs of DIC, hepatoencephalopathy, or severe bleeding. Recovering dogs should undergo periodic serum chemistry profiles and liver function testing (e.g. bile acid assay). Also screen dogs with a history of aflatoxin ingestion but no clinical signs for biochemical or hematologic abnormalities because clinical signs of aflatoxicosis are not always immediately apparent, and early treatment may decrease morbidity and mortality.13,16,18
LONG-TERM PATIENT OUTCOME
Dogs with aflatoxicosis can develop chronic liver disease and may be prone to developing neoplastic hepatic disorders. Hepatocarcinogenicity has been documented as a sequela to aflatoxin exposure in people, but no information is available regarding carcinogenicity in dogs.11
Canine aflatoxicosis occurs from ingestion of aflatoxin-contaminated foods. Aflatoxins cause severe hepatic damage that frequently results in liver failure. The mainstays of treatment are hepatoprotective nutraceuticals, fluid therapy, blood component therapy, vitamin K1, antiemetics, and gastrointestinal protectants. While aflatoxicosis is fatal in most patients with overt signs of intoxication, some dogs may recover slowly with long-term care.
The author wishes to thank Lisa Murphy, VMD, DABT, and Meredith Daly, VMD, for their contributions to this article.
"Toxicology Brief" was contributed by Eva Furrow, VMD, Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104-4912. The department editor is Petra Volmer, DVM, MS, DABVT, DABT.
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